research article experimental study of darrieus-savonius ... · internationaljournal of rotating...

Research ArticleExperimental Study of Darrieus-Savonius Water Turbine withDeflector: Effect of Deflector on the Performance

Kaprawi Sahim, Kadafi Ihtisan, Dyos Santoso, and Riman Sipahutar

Mechanical Engineering, Sriwijaya University, Jalan Raya Palembang-Prabumulih Km 32, Indralaya 30662, Indonesia

Correspondence should be addressed to Kaprawi Sahim; [email protected]

Received 21 October 2013; Accepted 8 January 2014; Published 20 February 2014

Academic Editor: Tariq Iqbal

Copyright © 2014 Kaprawi Sahim et al.This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The reverse force on the returning blade of a water turbine can be reduced by setting a deflector on the returning blade side of arotor. The deflector configuration can also concentrate the flow which passes through the rotor so that the torque and the powerof turbine can be considerably increased. The placing of Savonius in Darrieus rotor is carried out by setting the Savonius bucket inDarrieus rotor at the same axis. The combination of these rotors is also called a Darrieus-Savonius turbine. This rotor can improvetorque of turbine. Experiments are conducted in an irrigation canal to find the performance characteristics of presence of deflectorand Savonius rotor in Darrieus-Savonius turbine. Results conclude that the single deflector plate placed on returning blade sideincreases the torque and power coefficient. The presence of Savonius rotor increases the torque at a lower speed, but the powercoefficient decreases. The torque and power coefficient characteristics depend on the aspect ratio of Savonius rotor.

1. Introduction

Exploitation of renewable energies has been intensified tomeet the electricity needs when there is a potential sourceof hydropower. This kind of energies will decrease theenvironmental problems of pollution. River’s flow, canal flow,irrigation flow, and tidal current still have many difficultiesto be extracted optimally. This is due to the low efficiencyof the energy technology in which the water turbines usedfor stream flow have to be developed. One of the turbinescommonly used is a helical type which gives the efficiency of35% [1]. However, if a large capacity of water flows, generatingpower depends on taking lots of flow rate into the turbine.

Different designs of water current turbines are availablefor the extraction of energy from the river water or canals.Based on the alignment of the rotor axis with respect to waterflow, two generic classes exist. They are horizontal and ver-tical axis turbine. The commonly used vertical axis turbinesare Darrieus, Gorlov, and Savonius turbine. Helical blade andDarrieus straight blade water turbine are commonly suitablefor extraction of kinetic energy of flowing water. Helicalblades have a smaller rate of pulsation [2] andmore favorablestarting characteristics than straight blades [3], but the bladesare very difficult to be constructed than the straight blade.

The turbine system should be a simple structure and witha good reliability to fulfil the power generation so that onecan construct by itself like for small-scale applications of localproduction of electricity [4]. The tip speed ratio (TSR) ofDarrieus turbine is high and that is why it rotatesmuch faster.The Darrieus type has a cost-advantage due to the simplestructure. Installing the Darrieus turbine in a narrow canalwith small clearance will increase the efficiency [5]. Savoniusturbine is another type of simple one. This turbine has theadvantage of self-starting torque, but the efficiency and theoperation speed are lower. Savonius design uses a rotor that isformed by cutting the flattener cylinder into two halves alongthe central plane and then moving the two semicylindricalsurfaces sideways along the cutting plane so that the cross-section resembled the letter “S” [6].

The efficiency of Savonius turbine can be increased bymaking configuration of single stage, two-stage, and three-stage rotor. These configurations have been tested in a waterchannel as it was conducted by Khan et al. [7]. The useof two deflector plates can increase significantly the powercoefficient of modified Savonius rotor as studied experi-mentally by Golecha et al. [8],where one deflector is placedon the returning blade side and the other is placed on theoutside of the advancing blade side. To improve the starting

Hindawi Publishing CorporationInternational Journal of Rotating MachineryVolume 2014, Article ID 203108, 6 pageshttp://dx.doi.org/10.1155/2014/203108

2 International Journal of Rotating Machinery

torque of the Darrieus turbine used for power generation, aDarrieus-Savonius turbine, composed of a Darrieus turbineand Savonius rotor, has been experimentally tested in anopen channel. The Darrieus-Savonius turbine has a highertorque at a lower speed than the solo Darrieus turbine, butthe efficiency is lower than the Darrieus rotor regardless ofSavonius bucket orientation (attachment angle). In Darrieus-Savonius turbine, the best attachment angle of Savoniusrotor is placed perpendicularly to the Darrieus rotor. Theoperational tip speed ratio of solo Savonius turbine is lessthan unity and the solo Darrieus is greater than unity. Thecombined turbine of Darrieus-Savonius has the operationaltip speed ratio greater than unity [9]. In Darrieus-Savoniusrotor, the bucket of semielliptic cylinders is used to improvethe torque at a low speed; however, the rotation and efficiencyare still lower than solo Darrieus turbine [10]. The usageof guide vane increases the performance of wind turbine ofDarrieus type as it was tested by Takao et al. [11].

From the literature survey, the effects of deflector and thebucket size on the performance have not been clarified so far.It is the reason, in the present paper, we propose to observeexperimentally the impact of the Darrieus-Savonius turbineusing a deflector plate and variable bucket dimensions on theperformance which is represented by limits of torque andpower coefficient. The combined Darrieus-Savonius turbineconsists of Darrieus and Savonius turbine in which theSavonius rotor is placed on the middle of Darrieus rotoron the same shaft. Savonius water turbine and cross-flowwater turbine can be improved by setting deflector aroundthe rotors because the deflector in upstream of the rotor canincrease torque and power coefficient.

2. Experimental Setup and Procedure

In Darrieus-Savonius turbine, the Darrieus turbine has beenused as a main device and the Savonius turbine as a startupdevice. They are permanently attached to the same axis. TheSavonius turbine is composed of two semicircular buckets.Figure 1 shows the setting of a deflector plate to the combinedrotor.The deflector plate placed upstream to the fluid flow onthe returning blade side acts as an obstacle to the flow comingtowards the returning blade. This reduces the negative orreverse torque on the returning blade. Experiments areconducted for two positions of the deflector plate on thereturning blade side of the combined rotor. In the firstposition, deflector is set at 𝛽 = 0∘ (without deflector) and insecond position, the deflector is set at 𝛽 = 9∘. The deflectorplate has length of 100 cm. Setting the second angle is toobtain the height of the deflector ℎ = (1/2)𝑅.The gap betweenrotor blade and end plate of the deflector was about 40mm.Figure 2 gives the dimension of the blade and bucket. Thetested Darrieus rotor with the diameter of 𝑑D = 400mmhas two straight blades with a chord length of 𝐶 = 125mmand length of 𝐿D = 600mm. We use blade profile of NACA0015 for the reason of small drag when compared to bladesof higher thickness.TheDarrieus-Savonius rotor has also twoSavonius buckets with the diameter of a semicircular cylinder𝑑S and length 𝐿S.

DeflectorReturning bucket

Blade

Advancing bucket

𝛽 = 9∘h

Figure 1: Darrieus-Savonius rotor.

C

LD = 600mm

(a) Blade

dS

LS = 200mm

(b) Semicircular Bucket

Figure 2: Geometrical blade and bucket.

One of the purposes of the study is to reveal the influenceof a swept area of Savonius bucket. For that reason, we choseonly two buckets of diameter 𝑑S = 40mm and 63mm inDarrieus-Savonius turbine to be tested. The aspect ratio ofSavonius rotor is defined by a ratio of the length of bucketand the total width or diameter of two buckets. In this case,the total diameter of buckets is twice the diameter of eachbucket. The above dimensions of the bucket give the aspectratio of 𝐴 r = 2.5 and 1.58. It is noted that higher aspect ratiorepresents a smaller swept area (smaller 𝑑S). The bucket ismade of polyvinyl chloride (PVC) material and the blade ofDarrieus rotorwasmade ofwoodwith smooth surface. In thisstudy, solidity of Darrieus rotor is kept constant at 0.2 and itis calculated by the following expression:

𝜎 =𝐶 ⋅ 𝐵

𝜋 ⋅ 𝑑D. (1)

The turbine axis of diameter 14mm is held by two ballbearings.

The torque of the rotor was measured by a rope brakedynamometer with nylon rope of diameter 2mm. The waterturbine was first revolved without any loads and at thiscondition, the revolution was maximum. The loads weregradually added to the revolution by adding a certain masson weighing pan; then the rotation and the load on springscale were measured until the water turbine was completelystopped (overload).Themass on weighing pan wasmeasuredby digital scale with accuracy 500 gram/0.01 gram, while

International Journal of Rotating Machinery 3

(2)

(4)(5)

(6)

(1)

(3)

Darrieus blade(1)(2)(3)

(4)(5)(6)

Savonius bucketWeighing fan

RopeBrake wheelTubular spring scale

Figure 3: Schematic diagram of the test setup for the Darrieus-Savonius turbine.

the rotational speed of the rotor was measured using a lasertachometer.The same procedures of measurement were usedfor the other configurations of the combined turbine. Thecurrent meter flowatch FL-03 with accuracy 2% was used tomeasure the free stream velocity. The tubular spring scale ofrange 0–50N was used to measure the force (see Figure 3).

The rotational speed of the rotor is represented bydimensionless parameter tip speed ratio (TSR or 𝜆). TSR iscalculated by the following relation:

𝜆 =𝑈

𝑉. (2)

The effective torque, T, is calculated by

𝑇 = 𝑔 ⋅ (𝐹S − 𝐹W) ⋅ (𝑑𝑏+ 𝑑r2) [Nm] . (3)

Torque coefficient is obtained from the following relation:

𝐶𝑇=𝑇

𝑇𝑖

, (4)

where 𝑇𝑖is available torque; 𝑇 is effective torque.

The performance of the turbine is given by the powercoefficient 𝐶

𝑃and torque coefficient 𝐶

𝑇. These coefficients

represent the produced energy of the turbine as part of thetotal water energy passing through the swept area of theturbine rotor. This area equals the frontal area of the turbinegiven by the height times the diameter. As the blades performa complete circle, the blades in the downstream part of

Figure 4: Irrigation canal for turbine test.

the turbine are influenced by the wake resulting from theupstream blades. The available power in water flow is givenby

𝑃 =1

2𝜌𝐴𝑉3. (5)

Power coefficient is then defined by

𝐶𝑃=𝑃S𝑃. (6)

The aspect ratio of Savonius bucket is defined by the relationas follows [6]:

𝐴𝑡=𝐿S𝐷S. (7)

The experiments were carried out in an irrigation canalin Lubuk Linggau, Indonesia. The water velocity of the freestream is constant at 0.71m/s due to the irrigation systemof overflow in the main river. Water flow is coming fromriver in which the flow is diverted into the irrigation canalby using a sluice gate. The canal has 2.2m width and 0.8mdepth (Figure 4). The turbine rotor was set in the middle ofthe canal to avoid the effect of the boundary layer of flow nearthe canal wall.

3. Results and Discussion

Figure 5 shows the influence of aspect ratio of Savonius rotorson coefficient of torque and power of Darrieus-Savoniusturbine without deflector, which are compared to the soloDarrieus rotor. The torque of coefficient of solo Darrieusturbine has a peak value of 0.133 at 𝜆 = 1.06, while theDarrieus-Savonius turbine with 𝐴

𝑡= 2.5 has a torque

coefficient of 0.126 at 𝜆 = 0.88. This combined turbineshows a lower torque coefficient than solo Darrieus rotorand when the aspect ratio decreases (smaller swept areas),the coefficient becomes much lower. On the left part of thecurves, there are no data points because of the overload ofturbine with higher load and the rotor stops to run.

The performance of Darrieus-Savonius rotor is alsoshown by Figure 5(b) which presents the relation betweenthe power coefficients and tip speed ratio. It is observedthat solo Darrieus rotor has peak power coefficient of


0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.0 0.5 1.0 1.5 2.0 2.5

Solo DarrieusDarrieus-Savonius turbine with At = 2.5Darrieus-Savonius turbine with At = 1.58

CT

𝜆

(a) Torque coefficient

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.0 0.5 1.0 1.5 2.0 2.5


CP

𝜆

(b) Power coefficient

Figure 5: Characteristics performance of Darrieus-Savonius turbine without deflector.

0.00

0.04

0.08

0.12

0.16

0.20

0.0 0.5 1.0 1.5 2.0 2.5


𝜆

CT

(a) Torque coefficient

0.00

0.05

0.10

0.15

0.20

0.25

0.0 0.5 1.0 1.5 2.0 2.5

Cp

𝜆


(b) Power coefficient

Figure 6: Characteristic performance of Darrieus-Savonius turbine with deflector.

0.149 when compared to the Darrieus-Savonius rotor. Forcombined turbine with 𝐴 r = 2.5 (smaller frontal areaof the bucket), the peak power coefficient is 0.134 and fora smaller aspect ratio, the coefficient is much lower. Bothtypes of the combined turbines have a lower coefficient ofpower than the solo Darrieus. It is obvious that the biggerthe Savonius bucket in combined turbine, the smaller thecharacteristics of performance, and the bigger bucket makesa higher drag force and reverse force because the shear stressbecomes considerably significant. The convex surface side ofthe bucket may experience high skin friction drag and theconcave side may experience the induced drag force. These

parameters cause the deceleration of turbine rotation. Theprofile of torque and power coefficient shifts to the left partwhich shows that the rotation of Darrieus-Savonius turbineis smaller than the solo Darrieus turbine.

The performances of the turbines with deflector arepresented in Figure 6. The torque profiles are similar to thetorque of turbine without deflector (Figure 5(a)) in which thesolo Darrieus turbine has a higher torque for high rotation.We observe that using the deflector, all turbines have highertorque compared to turbines without deflector. At peakcondition, the increase is considerably significant and it isabout 30% for solo Darrieus and 40% for Darrieus-Savonius

International Journal of Rotating Machinery 5

turbine with𝐴𝑡= 2.5. It seems that at a lower tip speed ratio,

the torque of Darrieus-Savonius rotor with𝐴𝑡= 2.5 is higher

than that of solo Darrieus. There is a little improvement ofturbine torque at a lower speed.

The increase of the torque coefficient is followed by theincrease of the power coefficient as shown by Figure 6(b).At condition of peak value, the power coefficient increases.For solo Darrieus turbine, the increase achieves 41% and forDarrieus-Savonius turbine with 𝐴

𝑡= 2.5, the increase is

about 30% or the maximum value increases from 0.149 to0.211 for solo Darrieus turbine and from 0.134 to 0.171 forDarrieus-Savonius turbine with 𝐴

𝑡= 2.5.

We observe further, the presence of the deflector has avery little influence upon the tip speed ratio of Darrieus-Savonius and soloDarrieus turbine as shown by Figures 5 and6. In fact, the number of rotations of turbine with deflectorincreased but when it is transformed into the dimensionlesstip speed ratio. It becomes little changes in variation of 𝜆. Bysetting the deflector at a certain angle, the free stream velocityincreased due to concentration of flow and this makes theincrease of turbine rotation so that it may be constant asstated by the definition of the tip speed ratio. The presenceof deflector, the coefficient of torque, and coefficient of powerincrease significantly as shown by Figures 5 and 6.

We observe all configurations of Darrieus-Savonius tur-bine containing Savonius rotor and that the rotational speedbecomes lower than solo Darrieus rotor. The characteristicperformances decrease from the solo Darrieus performance.It may be caused by geometric of a bucket of Savoniusrotor. The higher bucket dimensions decrease the rotorperformance and it is due to higher skin friction drag ofviscous flow water on the outer surface of the convex bucketin all complete rotations of the rotor. The other part, theconcave side of bucket section, produces induced drag whenrotating at returning side.

4. Conclusions

From the above discussions, it is concluded that installingof Savonius in Darrieus rotor without deflector makes theDarrieus-Savonius rotor have lower performance character-istics than solo Darrieus rotor, but the torque has a littleincrease. By setting a deflector plate in upstream rotor ofDarrieus-Savonius turbine, the power coefficient character-istics and torque coefficient characteristics increase signif-icantly. In Darrieus-Savonius, the aspect ratio of Savoniusbuckets affects the turbine performances because a higherperformance is obtained for a higher aspect ratio.

Abbreviations

𝐴𝑡: Aspect ratio

B: Number of bladesC: Chord length of blade (m)𝐶𝑝: Power coefficient𝐶𝑇: Torque coefficient𝑑𝑝: Diameter of brake wheel (m)𝑑D : Diameter of Darrieus rotor (m)

𝑑r : Rope diameter (m)𝑑S : Diameter of semicircular bucket (m)𝐷S : Total diameter of Savonius rotor (= 2𝑑S)𝐹𝑆: Load on spring scale (kgf)𝐹W : Load on weighing pan (kgf)g: Gravitation force (m/s2)h: Height of deflector position (m)𝐿D : Length of Darrieus blade or rotor (m)𝐿S : Length of Savonius bucket or rotor (m)P: Water power (Watt)𝑃S : Shaft power (Watt)R: Radius of Darrieus rotor (m)T: Torque (Nm)U: Tangential velocity of Darrieus rotor (m/s)V : Free stream velocity (m/s).

Greek Symbols

𝛽 : Deflector angle (∘)𝜆 : Tip Speed Ratio𝜎 : Solidity.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgment

This paper is supported by the BOPTN Fund of SriwijayaUniversity.

References

[1] A. N. Gorban’, A. M. Gorlov, and V. M. Silantyev, “Limits of theturbine efficiency for free fluid flow,” Journal of Energy ResourcesTechnology, vol. 123, no. 2-4, pp. 311–317, 2001.

[2] J. D. Winchester, S. D. Quayle, and S. D. ”, “Torque ripple andvariable blade force: a comparison of darrieus and gorlov-typeturbines for tidal stream energy conversion,” in Proceedings ofthe 8th European Wave and Tidal Energy Conference, Uppsala,Sweden, 2009.

[3] M. Shiono, K. Suzuki, and S. Kiho, “Output characteristicsof darrieus water turbine with helical blades for tidal currentgenerations,” in Proceedings of the 12th International Offshoreand Polar Engineering Conference, pp. 859–864, Kitakyushu,Japan, May 2002.

[4] J.-L. Menet, “A double-step Savonius rotor for local productionof electricity: a design study,” Renewable Energy, vol. 29, no. 11,pp. 1843–1862, 2004.

[5] D. Matsushita, K. Okuma, S. Watanabe, and S. Furukawa,“Simplified structure of ducted Darrieus type hydro turbinewith narrow intake for extra low head hydropower utilization,”Journal of Fluid Science and Technology, vol. 3, no. 3, 2008.

[6] M. J. Alam and M. T. Iqbal, “A low cut-in speed marine currentturbine,” Journal of Ocean Technology, vol. 5, no. 4, pp. 49–62,2010.

[7] M. N. I. Khan, M. Tariq Iqbal, M. Hinchey, and V. Masek,“Performance of savonius rotor as a water current turbine,”Journal of Ocean Technology, vol. 4, no. 2, pp. 71–83, 2009.


[8] K. Golecha, T. I. Eldho, and S. V. Prabhu, “Investigation onthe performance of a modified savonius water turbine withsingle and two deflector plates,” in Proceedings of the 11thAsian International Conference on FluidMachinery, IITMadras,Chennai, India, November 2011.

[9] Y. Kyozuka, “An experimental study on the Darrieus-Savoniusturbine for the tidal current power generation,” Journal of FluidScience and Technology, vol. 3, no. 2, 2008.

[10] S. Kaprawi, D. Santoso, andA. Radentan, “Performance of com-bined water turbine with semielliptic section of the savoniusrotor,” International Journal of Rotating Machinery, vol. 2013,Article ID 985943, 5 pages, 2013.

[11] M. Takao, H. Kuma, T. Maeda, Y. Kamada, M. Oki, and A. AndMinoda, “A straight-bladed vertical axis wind turbine with adirected guide vane row—effect of guide vane geometry on theperformance,” Journal of Thermal Science, vol. 18, no. 1, pp. 54–57, 2009.

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